1. Field of the Invention
The present disclosure relates to automatic detection of electric threshold conduction of lighting load and its application.
2. Description of Related Art
A dimmable lighting apparatus installed outdoors or indoors always comprises lighting bulb(s) with a dimmer circuit. The dimmer circuit is used to control light intensity of electric bulb commonly for energy-saving, creating an aesthetic atmosphere or providing security lighting purposes. In general, the dimmer connected in-series with alternating-current (AC) lighting bulb and AC power is a conductive phase angle control circuit composed of a triac device and a trigger means. Dimmer circuit of this art is disclosed for example in U.S. Pat. No. 5,598,066, where an analog circuitry is used to implement a two-level security light. The analog dimmer circuitry employs a triac device which can be bi-directionally triggered into conduction for a time period during positive and negative half-cycle of AC power. The conduction time period manageable by trigger time point is here referred to as conductive phase angle of AC half-cycle. In the two-level security light, the triac device controls conductive phase angle to regulate electric current flowing through a lighting bulb for a long or a short time period during each AC half-cycle, to respectively render the lighting bulb emitting a high or a low level light intensity. An improved analog version based on conductive phase angle control is further disclosed in U.S. Pat. No. 6,225,748 B1 in which a short conductive period controlled by the triac device can be continuously varied in a small range such that the low level light intensity of the AC lighting bulb is adjustable.
Whenever an user trying to replace a burned out AC lighting bulb in dimmable lighting apparatus, the common problem he always faces is the requirement of having to buy exactly the same lighting bulb that was previously used; otherwise the dimming would not perform well, for instance, the light bulb flickers or the bulb illumination cannot be smoothly varied by a dimming operation. In view of such inconvenience, a universal dimmer that can be equally operable for different type bulbs may be highly welcome. Effort in this direction is for example in U.S. Pat. No. 8,198,820 B2 where a dimmer circuit operable on both incandescent and fluorescent lamps is disclosed. This dimmer circuit uses controller circuitry to automatically detect whether an incandescent or a fluorescent bulb is connected with the dimmer, and then to conduct corresponding dimming work. There was no mention about how the dimmer circuit can manage the variation of threshold voltage among different brands of fluorescent bulbs.
The dimmers of prior art are generally constructed by analog electric circuitry that may be complex in system structure and lacks flexibility due to its hardware nature. In U.S. Pat. No. 8,310,163 B2 a digital-type dimmer circuit is disclosed; the circuit uses programmable digital device, for example, microcontroller, to control the conductive phase angle of a triac device, and therefore to manage the illumination intensity of a lighting bulb. A specialty of this art is using program codes to control illumination. Thus, the circuit construction is much simpler than that of the analog-type dimmer. Further, the lighting control can be done with high accuracy defined by the program codes. If extra dimmer function is desired, it needs solely to modify program codes such that circuit hardware remains unchanged.
FIG. 1A shows block diagram of a microcontroller-based dimmer that works on the principle of conductive phase angle control. The dimmer of FIG. 1A comprises a bidirectional control switch 11, a microcontroller (MCU) 12, a zero-crossing-point detector 13 and an external control unit 14. The bidirectional control switch 11 is preferable a triac. For lighting control, an AC lighting bulb or a lighting load 2 is connected in-series to the bidirectional control switch 11 and AC power source 3. The external control unit 14 is used as a communication interface between microcontroller 12 and user. The microcontroller 12 and the zero-crossing-point detector 13 build a trigger circuit. In accordance with user's instructions sent from the external control unit 14, the trigger circuit generates in each AC half-cycle a trigger signal to turn on the bidirectional control switch 11, such as to control the conductive phase angle or the conductive time period in each AC half-cycle. Corresponding to the conductive time period, an average AC electric power is delivered through the bidirectional control switch 11 to the lighting load 2 to generate light with intensity in proportion to the average AC power.
With technology advance of lighting bulbs, the dimmer design may encounter difficulties caused by different properties of lighting bulbs produced by new technology. Technically, the lighting bulbs in daily use are AC electric loads with two terminals. FIG. 1B illustrates respectively in drawings (a)-(c) three non-incandescent light bulbs for directly plugging in AC socket, each has terminal voltage V and terminal current I as indicated. They are: (a) screw-in compact fluorescent bulb, (b) two-terminal ACLED module, having two light-emitting diodes (LEDs) connected in-parallel and arranged with reverse polarity, and (c) screw-in LED bulb. FIG. 1B (d) illustrates a built-in circuit in a screw-in LED bulb. Please refer to drawings (c) and (d) of FIG. 1B. Generally, a screw-in LED bulb is composed of a full-wave rectifier D1-D4 and a plurality of LEDs. The LEDs are connected in series and attached to the output port b-b′ of the full-wave rectifier. The input port a-a′ of the full-wave rectifier is either directly or via a voltage reduction branch C1-R connected to an AC power source. The capacitor C2 and the Zener diode ZD are used to maintain a quasi constant voltage applied on the series-connected LEDs. The LED, like a common diode, needs a cut-in voltage to begin conducting electric current. It can infer from such a diode property that a threshold voltage in proportion to the number of the LEDs is required to turn on the series-connected LEDs. Thus, except that the terminal voltage V of the screw-in LED bulb exceeds the threshold voltage, the series-connected LEDs are cut-off such that the terminal current I is zero. Similar electric conduction behavior resulted from threshold voltage is also observed in fluorescent lamp and ACLED module despite different constructions.
Please refer to FIG. 2. The three lighting bulbs of FIG. 1B can be equally represented by a two-terminal electric element with electric property as illustrated respectively in drawings (a) and (b) of FIG. 2. The electric element has two terminals A and A′. When measuring voltage V and current I at the terminals A-A′, the electric element reveals a nonlinear relation between current I and voltage V as shown in (b) of FIG. 2. The terminal current I begins to increase sharply when the terminal voltage V varies along the voltage axis and exceeds a threshold voltage Vt for both positive and negative polarity. By contrast, if the terminal voltage is limited in a domain ranging from −Vt to Vt, the terminal current I is nearly zero. It means that an AC lighting load having such nonlinear I-V curve can conduct electric current and emit light only when an instant AC voltage is greater than the threshold voltage Vt; otherwise, the lighting load is turned off completely. The threshold voltage Vt of new-generation lighting bulb such as LED is large, for instance, Vt>80 V in ACLED module and screw-in LED bulb. In comparison, a conventional incandescent bulb has a relatively linear I-V characteristic; it emits light starting instantly from zero terminal voltage.
Reference is made to FIG. 3. FIG. 3 shows signal waveforms (a)-(c) for dimmer operation specifically concerned with dimmer of FIG. 1A and the lighting load of FIG. 2. The waveform (a) represents sinusoidal AC voltage variation of an AC power source 3. The waveform (a) of AC voltage has amplitude Vm and half-cycle period T. For easy explanation, consider only the positive half-cycle of the AC voltage. Along the time axis, the first zero-crossing point is indicated as t=0 and the second zero-crossing point is as t=T. If a lighting load 2 having threshold voltage Vt is connected via a bidirectional control switch 11 to the AC power source 3, it remains cutoff respectively for the time periods ranging from t=0 to t=tD0 and from t=T−tD0 to t=T, where tD0=(T/π) sin−1 (Vt/Vm), no matter whether the bidirectional control switch 11 is triggered or not. Accordingly, t=tD0 is the time point measured from the first zero-crossing point when the instant AC voltage is equal to the threshold voltage Vt; the parameter tD0 defined on the time axis is the time phase of the threshold voltage Vt. In other words, only in a time period from t=tD0 to t=T−tD0 the AC voltage exceeds the threshold voltage Vt with sufficient magnitude to hold the lighting load 2 in conduction state. Similar description can be equally applied to the negative half-cycle of the AC voltage.
The waveforms (b) and (c) in FIG. 3 represent two different voltage signals generated from microcontroller 12 shown in FIG. 1A, for triggering and bringing the bidirectional control switch 11 into conduction. The waveforms (b) is a pulse-width-modulation (PWM) signal synchronizing with the waveform (a) of AC voltage; the PWM signal uses a high voltage to turn on the bidirectional control switch 11 in each AC half-cycle, such that its front edge with a time delay tD counted from the zero-crossing point of the AC voltage can be considered as the trigger time point to turn on the bidirectional control switch 11. The bidirectional control switch 11 has thus a conductive time period given by T−tD manageable by varying the time delay tD in each AC half-cycle.
Because of the series connection as shown in FIG. 1A, the lighting load 2 emits light under condition that both the lighting load 2 and the control switch 11 conduct electric current at the same time. By examining the waveforms (a) and (b) in FIG. 3, a common time domain exists for both components 2 and 11 being conductive. This common time domain demands that the PWM signal should be generated with its time delay tD limited in a range: tD0<tD<T−tD0. An AC power is then effectively transmitted via the bidirectional control switch to the lighting load for a time period of T−tD0−tD in each AC half-cycle. The illumination power of the lighting load is therefore determined by the conductive time period T−tD0−tD. In this context, the illumination intensity of a lighting load is manageable by varying the time delay tD of the PWM signal generated by microcontroller circuitry in each AC half-cycle.
From the above account, the threshold voltage Vt of the lighting load imposes a constraint on generating PWM signal to turn on both the lighting load and the control switch at the same time. The lighting load with threshold voltage Vt has two non-conductive phase zones adjoining zero-crossing points in each AC half-cycle. The PWM signal for conductive phase angle control should take these two non-conductive phase zones into account and keep technically its front edge with a time lag of tD after the zero-crossing point of the AC voltage in a time range: tD0<tD<T−tD0, to ensure stable light dimming operation. This constraint has been disclosed in U.S. Pat. No. 8,310,163 B2. Because of different threshold voltages, the dimmer designed for a lighting load would not be likewise operable for another lighting load. Usually, as a safe measure to ignore the condition tD0<tD<T−tD0, the dimmer designer takes deliberately a large time delay tD to mitigate any effects caused by the threshold voltage. For instance, the waveform (b) in FIG. 3 is the PWM signal with a time delay tD roughly equal to T/2, where an instant AC voltage, namely, the AC amplitude Vm, is greater than the threshold voltage Vt to secure a smooth dimming or flickering-free state of the lighting load; under this condition the lighting load delivers 50% intensity of the full power illumination. The waveform (c) in FIG. 3 is a constant high voltage in such a way to automatically bring both the control switch 11 and the lighting load 2 into conduction when the instant AC voltage surpasses the threshold voltage Vt in each AC half-cycle; under this condition the lighting load delivers 100%-intensity of illumination. The circuit design using trigger signals like waveforms (b) and (c) in FIG. 3 is employed vastly in conventional dimmers. However, if precise and special illuminations are demanded, such as an illumination with adjustable intensity level, or an illumination of gradually changing brightness between two predetermined levels (soft start/soft end), the conventional dimmers would not work with success if lighting loads are different types and brands manufactured from different factories.
In summary, the non-conductive phase zones caused by the threshold voltage of a lighting load are crucial to dimmer operation. Therefore, the dimmer circuit designed for operating one specific brand of lighting load may not work well for operating other brands, due to the variations of the threshold voltage and other electric parameters. In dimmer design, the engineers take usually a strategy to secure that the triac device is triggered into conduction at a time phase when the instant AC voltage safely exceeds the threshold voltage. A sufficient time phase cushion is thus designed to accommodate variations of threshold voltages among different light bulbs. However, such an endeavor may be at the cost of the limited dimming capability ranging generally from nearly 50%- to 100%-brightness of the full power illumination. The attempt to achieve lower than 30% of full power brightness would cause non-performing or flickering of the lighting load.
Another application associated with a dimmable lighting apparatus is to add extra function to make light intensity of the low or the high level illumination adjustable. The two-level lighting management disclosed in U.S. Pat. No. 5,598,066 is restricted to a fixed low level illumination wherein an accent light of low level intensity is automatically turned on at dusk and a high level illumination is turned on upon detection of a motion intrusion. This low level intensity is usually preset by the dimmer manufacturer to yield nearly 50% intensity of the full power illumination. However, it is commonly the case that the end user can best determine the adequacy of the low level lighting for their living environment to create cos y atmosphere and beautiful night view. At 50% intensity for low level the security alert function, the aesthetic night view and the energy saving all are compromised for nothing meaningful. Although in U.S. Pat. No. 6,225,748 B1 an analog circuitry has been disclosed for adjusting low level light intensity, this obsolete technology cannot define precisely the range of adjustability due to the fundamental constraints of analog circuitry. This analog circuitry is also quite cumbersome in performing a simple function which can be easily accomplished by a software program with simple circuit design. An advanced circuit solution other than the prior art to offer adjustability of low or high level illuminations is definitely needed.